Coming Up For Air: 100 Million Years of Ocean Biology


Published Online: 18th February 2018



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George Cuvier was a young man at the Storming of the Bastille in the summer of 1789. It was under the shadow of the French Revolution that he developed the concept of ‘catastrophism’. In the midst of the radical political changes that were engulfing Europe, Cuvier speculated that the Earth itself had undergone radical, rapid and irreversible changes that defined the boundaries seen between consecutive layers of rock. Cuvier was one of the first naturalists to advocate the idea of biological extinction – the idea that species could have permanently disappeared from the face of the Earth, leaving only fossil remains behind. Over 200 years later, palaeontologists now recognise not only the reality of extinction, but also that the vast majority of all species which have ever existed are now extinct. This sobering fact provides lessons which could prove important for the continued prosperity of our own species.

The rate of at which species go extinct is not constant throughout the Earth’s history. Large, structurally complex animals have existed on the Earth for around 541 million years; in that time there have been five major and tens of minor mass extinction events – relatively short periods of time in which large portions of all life became extinct. Most well-known among these is the end-Cretaceous extinction of 66 million years ago, which wiped out the non-avian dinosaurs and set the stage for the rise of mammals.

My research concerns one of the more ‘minor’ extinction events. Cast your mind back to 94 million years ago, to a warmer world where much of the European continent is submerged beneath the sea. On land, the dinosaurs are still in their prime. The first flowering plants recently evolved and have been joined by the earliest ants to ever exist. Meanwhile, massive reptiles such as the dolphin-like ichthyosaurs are the dominant predators in the ocean. Over the course of the next five-hundred-thousand years several groups of marine reptiles, including the ichthyosaurs, along with a substantial portion of all marine invertebrates, will become extinct.

 

For tens of millions of years, ichthyosaurs were among of the largest predators of the world’s seas – the largest species being more than 10m long. Having experienced a protracted decline, the last ichthyosaurs finally became extinct during OAE-2. This painting is an early 20th century interpretation by Heinrich Harder.
For tens of millions of years, ichthyosaurs were among of the largest predators of the world’s seas – the largest species being more than 10m long. Having experienced a protracted decline, the last ichthyosaurs finally became extinct during OAE-2. This painting is an early 20th century interpretation by Heinrich Harder.

 

Just like animals which dwell on the land, sea creatures require oxygen to breathe – most of them extract from seawater directly. During this extinction event, vast swathes of the ocean became almost completely deprived of oxygen – an episode known as an oceanic anoxic event (OAE). This particular OAE is (uncreatively) known as OAE-2 and took place during a geological period called the Cretaceous. The causes of OAEs are debated, but this one was probably linked to major increase in the release of carbon dioxide and other materials from volcanoes.

Fossils are the window through which we view the history of life on Earth. By comparing the number and types of animals that existed before OAE-2 to those which existed after, we can try and understand the reasons why some species went extinct while others managed to survive despite the major environmental change.

 

A variety of invertebrate fossils from the Cretaceous currently housed at the Oxford Museum of Natural History, some of which belong to groups that are included in my research. Large-scale studies of past life often use shelly invertebrates because of their relatively good fossil record.
A variety of invertebrate fossils from the Cretaceous currently housed at the Oxford Museum of Natural History, some of which belong to groups that are included in my research. Large-scale studies of past life often use shelly invertebrates because of their relatively good fossil record.

 

As any experienced fossil hunter will tell you, fossils are not very common in rocks. It would take an entire lifetime to gather enough fossils so that you could gain a good understanding of the fullness of biological diversity for any single point in geological time. A solution to this problem has emerged in the form of the Paleobiology Database (PBDB). PBDB is a free, open-access website that allows palaeontologists to upload data on any fossils they have found when conducting their research – over 1.3 million individual fossil occurrences have been listed in PBDB at the time of writing. Any armchair palaeontologist with an internet connection can go onto the website to generate a list of all known species that existed for a given period of geological time. The site even has a built-in ‘Navigator’ tool that makes it easy for anyone to create beautiful graphs showing changes in biodiversity over hundreds of millions of years. PBDB has revolutionised our ability to understand large-scale changes in biology over the history of the Earth.

I have been using data from PBDB to try and understand whether particular animal characteristics – diet, habitat and movement speed – had an impact on whether or not a species survived OAE-2. The hope is that if we can figure out what animals were more likely to go extinct in the past, we will be able to predict what animals might be at the greatest risk of going extinct in the future.

Being able to make these predictions is especially important in the modern world, as ocean-dwelling animals are beginning to feel the impact of anthropogenic climate change. Ocean acidification and temperature increases are already significantly affecting ocean biology. Oxygen-minimum zones (OMZs), where there is insufficient dissolved oxygen to support large animals, occupy only a relatively small volume of the modern ocean. Future warming will cause changes in the structure of the ocean that may lead to the expansion of OMZs, and the total amount of oxygen dissolved in the ocean is predicted to decrease up to 7% by 2100.

 

Oxygen concentrations in the modern ocean are typically measured using oxygen sensors attached to an instrument called a CTD (conductivity-temperature-depth), which is deployed from a ship and can take measurements down to several kilometres depth. This particular CTD setup was deployed from the RV Atlantic Explorer on a survey in the Sargasso Sea, close to Bermuda.
Oxygen concentrations in the modern ocean are typically measured using oxygen sensors attached to an instrument called a CTD (conductivity-temperature-depth), which is deployed from a ship and can take measurements down to several kilometres depth. This particular CTD setup was deployed from the RV Atlantic Explorer on a survey in the Sargasso Sea, close to Bermuda.

 

The potential collapse of ocean food chains is also seriously bad news for humans, as billions of people worldwide are dependent on seafood as a source of protein and micronutrients. Recent declines in fish populations will only be exacerbated by the future expansion on OMZs. Particularly hard-hit will be those poorer countries which catch fish from areas that are already affected by periodic anoxia, including Namibia, the Philippines, and India. Global fisheries are faced with the seemingly impossible challenge of feeding a growing human population from a drastically diminishing food source.

The near-incomprehensible scale of geological time offers us an invaluable insight into our possible future. If we are to be adequately prepared for the challenges ahead it is vital that we take stock of the lessons that quite literally lie beneath our feet.

 

– Matt Sutton

 


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